The Zaku I is Zeon's first combat-worthy mobile suit. Although designed as a front line combat unit, the Zaku I was soon replaced by the improved MS-06F Zaku II and would be replaced as the mainstay of Zeon's mobile suit forces. Early during the One Year War of UC 0079, most Zaku Is were relegated to menial construction and supply duties, although a few did continue to serve in combat roles, such as the gassing of colony populations, attaching booster engines to space colonies to hurtle them towards Earth, and so on.

The Zeonic Company began trial production of the MS-05A model in May of UC 0075, producing a total of 27 units. In November of the same year, a mobile suit training battalion was formed to gather performance data and practice combat maneuvers. Based on feedback from this training battalion, a modified MS-05B version was created for mass production, and approximately 800 of these were produced. It wasn't until the last months of the war that Zeon, desparate after facing losses in Operation Odessa and Operation Star One, would move the old Zaku Is back to the front lines to reinforce their faltering main forces.

Source: Gundamproject.com, HG-UC kit manual